There has been a desire for an instrument to monitor constituents in an animal or human organ non-invasively. A particular example is monitoring of oxygen level in the brain which is particularly important, for example during surgery, where a significant number of patients come out of the anesthesia with various degrees of and sometimes permanent brain function deficiency. It is believed that in a significant portion of such cases, lack of sufficient oxygen to the brain is the cause of such deficiencies. Thus, the ability to accurately monitor oxygen level in the brain directly, rather than through indirect methods such as a pulse oximeter placed on another portion of the body, would have obvious advantages including non-invasiveness, immediate and timely results, and relative simplicity. Techniques to achieve such monitoring have involved passing near-infrared radiation through a cranium and analyzing the modified output radiation.
One known method is to pass radiation having several discrete wavelengths from laser diodes equal in number to the number of constituents to be measured, for example two wavelengths for oxygenated and deoxygenated hemoglobin. The radiation is modulated with radio frequency. The output modified by the brain is used to calculate changes in amplitude and phase which lead to determination of absorption coefficients at the different wavelengths. Simultaneous equations with these coefficients determine concentrations of the constituents of interest and the oxygen saturation which is the percentage of oxygenated to total hemoglobin.
Another method is to utilize continuous-wave radiation, in which output from a detector on a cranium is spectrally analyzed to yield oxygen saturation. Although a full spectrum is used, the analysis is based on modeling with either a small number of wavelengths or a few known constituents such as the oxy and deoxy hemoglobin and water.
Any such monitoring encounters difficulties resulting from the biological complexities of an organ such as a brain, compared with spectrometric instrumentation that ordinarily analyzes fluids that are readily probed, contained or flowing in a tube suitable for the instrument. Geometries of different subjects vary considerably and variations occur even within an individual. Further, tissues are not uniform. The radiation is scattered so that a path is not well defined. Signal to noise ratios for infrared radiation through solid material are generally low. Current methods for monitoring of craniums depend on theoretical or mathematical models that may be oversimplified or inaccurate. Thus there is a need for better accuracy and reproducibility.
Consequently, an object of the invention is to provide a novel method and means for monitoring constituents in an animal organ non-invasively, particularly oxygenated and deoxygenated hemoglobin in a brain.